FIG. 1 depicts certain elements of a refrigerant circuit for a vehicle heating, ventilation, and air conditioning (HVAC) system 100. As is known, such systems include a condenser 102, a drier/receiver 104, one or more evaporators 106, an expansion valve 108, and a compressor 110 which is typically driven by a power source such as the vehicle engine 112. Various conduits 114 inter-connect the described elements for transport of refrigerant, etc. therebetween.
As is known, many compressors 110 in use in modern motor vehicles are variable displacement compressors which provide the advantage of automatically varying their displacement capacities to meet air-conditioning demands, and also offer further advantages of smoother operation and improvements in vehicle fuel consumption compared to the fixed displacement compressors. FIG. 2 illustrates an externally-controlled variable displacement compressor (EVDC) 200 in side cross-section. Specific details of the structure of a variable displacement compressor 200 may vary. However, at a high level such compressors include a housing 202, a suction cavity 203, a suction port 204, a discharge cavity 205, and a discharge port 206. A solenoid 212 is provided to operate a displacement control valve or electronic control valve (ECV) 214 with a high side port 208 and a low side port 210 (this structure is replaced by a bellows in an internally controlled variable displacement compressor, which is otherwise structurally similar to the externally controlled variable displacement compressor 200 described herein). The ECV is operatively connected to a controller generally depicted as reference numeral 213. The controller 213 may also be configured to receive inputs from one or more temperature sensors, for example a sensor associated with the evaporator 106, a temperature sensor associated with an air discharge duct, and an ambient temperature sensor (depicted generally as temperature sensors 215a, 215b, 215c).
A control chamber 216 (also referred to as a crank case) houses a drive shaft 218 carrying a swash plate 220 which is in turn operatively connected to multiple sets of pistons 222. A base plate 224 carried by the drive shaft 218 is in turn operatively connected to the swash plate 220 by a pivot linkage 226. As the drive shaft 218 actuates linearly, the swash plate 220 alternately drives each piston 222 through a stroke cycle, whereby each piston stroke traverses between a minimum and maximum stroke limit. A suction reed valve 228 and a discharge reed valve 230 selectively place the control chamber 216 in fluid communication with, respectively, the suction cavity 203 and the discharge cavity 205 enabling fluid flow in through the suction port 204 and flow out through the discharge port.
Under normal compressor 200 engagement conditions, a bleed hole 232 connecting the control chamber 216 to the suction cavity 203 allows de-pressurization of the control chamber, while building up pressure at the top of the pistons 222 allowing the swash plate 220 to stroke up to maximum angle and leading to cooling performance.
During extended off times at moderate to elevated ambient temperatures, however, the control chamber 216 accumulates fluid (typically a liquid refrigerant and oil mixture) due to refrigerant migration into the relatively cooler compressor from the relatively warmer refrigerant subcomponents, primarily the condenser and the evaporator. On a next engagement or operation of the compressor 200, this fluid must be evacuated through the bleed hole 232 before depressurization of the control chamber 216 can occur and the compressor swash plate 220 can stroke up to enable cooling performance. As a result, a perceptible delay of up to one minute or more is experienced before the air flowing into a vehicle passenger cabin (not shown) is cooled by the evaporator 106. This results in user dissatisfaction.
To solve this and other problems, the present disclosure relates to methods for controlling and improving a variable displacement compressor cold-start which reduce compressor start-up delay, and consequently reduce the time required to provide a cooling airflow to a vehicle passenger cabin.